Smoking during pregnancy exerts an independent, adverse effect upon numerous reproductive outcomes, and thus the reduction in the prevalence of prenatal smoking has been a national priority for the past decade. Approximately a quarter of US women smoke prior to becoming pregnant, with a third of these smokers quitting prior to the start of prenatal care - and are referred to as Spontaneous Quitters (SQs). Several studies have documented that at least 25 percent of SQs relapse prior to delivery, and therefore the health of the mother and fetus is once again jeopardized due to tobacco exposure during pregnancy. To date, randomized trials testing various interventions have failed to reduce prenatal relapse with this group. This study proposes to develop a telephone counseling relapse prevention program based on the principles of motivational interviewing to address the needs of this unique group of recent quitters. The theoretically-grounded program will be developed during a formative assessment period consisting of in-depth interviews and focus groups with a representative sample of SQs. The effectiveness of the intervention will be tested under conditions of typical clinical practice among a diverse population of prenatal patients who are members of a large HMO (Southern California Kaiser- Permanente). A total of 480 SQs will be randomly assigned to either a) usual care -- consisting of provider advice which may be offered during prenatal visits and a self-help smoking cessation/maintenance booklet; or b) usual care + the experimental telephone-based counseling intervention. The principal dependent variable will be biochemically confirmed maintenance of cessation for the duration of pregnancy. If effective, the proposed intervention offers the opportunity to decrease the prevalence of prenatal smoking among the approximate 1 million US women who annually initiate prenatal care as prepregnancy smokers. Finally, as more than 75 percent of the women who stop smoking during pregnancy are SQs and given the high rate of postpartum relapse, learning about successful maintenance during pregnancy may aid intervention efforts to prevent the return to smoking after delivery.